In order to improve the aero-acoustic, vibratory, and aerodynamic performance of the blades of a main rotor for providing a rotorcraft with lift and propulsion, it is advantageous to have a tiltable flap along the trailing edge of each blade, referred to for convenience as a trailing edge flap.
The angle to which the flap can be tilted relative to the blade is of the order of ±10° at a frequency of about 30 hertz (Hz). In order to be effective, the angle of tilt varies actively over time as a function of numerous parameters, such as: the position of the blade around the axis of the main rotor, which position is known as its “azimuth”; the speed of rotation of the main rotor; and the pitch, flapping, and lag movements of the blade.
Given the dimensions of the blade and the stresses acting on the flap and the blade due to the flapping, lag, and pitch movements of the blade, it can readily be understood that there are difficulties to be overcome in order to obtain a flap that is reliable, lightweight, and capable of satisfying the expectations of the manufacturer and of users.
Patent document US 2002/0021964 discloses a first type of blade provided with a flap. The top and bottom portions of the leading edge of the flap are convex in shape while the central portion of the leading edge is concave in shape. The leading edge of the flap is thus ε-shaped.
In addition, the top and bottom portions are each connected to a respective actuator, while the concave central portion is placed against a bearing element secured to the blade. Consequently, by activating one or other of the actuators, the flap tilts about the bearing element so as to present the desired angle of inclination.
However, under the effect of the stresses exerted on the blade and the flap in flight, in particular those caused by flapping movements, there is a risk of the flap sliding along the bearing element which can lead to premature wear, or even, in a worst-case scenario, to the flap becoming jammed so as to become unusable and dangerous.
Furthermore, it can clearly be seen that the overall shape of the flap and more particularly of the leading edge of the flap is not optimum from an aerodynamic point of view, which might lead to disagreeable noise or vibration and to degraded aerodynamic performance.
Furthermore, patent document U.S. Pat. No. 6,454,207 discloses a second type of blade having a flap at its trailing edge.
The flap is secured to the blade by a longitudinal shaft, i.e. extending along the span of the blade, which passes through the flap. Consequently, the flap is suitable for pivoting about said longitudinal shaft, being moved by a hinged connecting rod secured to the pressure side of the flap, said connecting rod itself being controlled by an actuator. Part of the connecting rod thus lies outside the blade-and-flap assembly, which is bad for the aerodynamic performance thereof.
That second type of architecture in which the flap pivots about a shaft also presents the drawback of the flap possibly jamming in flight. It should not be lost from sight that the mechanical stresses exerted on the flap and the blade in flight are very high. Consequently, there is a non-negligible risk of the shaft becoming deformed, and as a result of the flap being prevented from tilting to the desired angle of inclination.
Finally, patent document FR 2 770 826 discloses a third type of blade fitted with a flap. The flap is no longer connected to the blade by a longitudinal shaft, but by two stub axles located at opposite ends of the flap. In addition, the flap is provided with a flexible arm that is positioned inside the blade, being connected via a lever to a set of two rotary motors acting on coaxial eccentrics. By means of those two rotary motors, the device controls the angled inclination of the flexible arm and thus of the flap.
Nevertheless, as for the second type of blade, the stub axles about which the flap pivots do not appear to present sufficient robustness to guarantee proper operation of the system over a reasonable length of time.